The invention relates to improved means and method for avoiding image formation on the seam of a photoconductive belt of an electrophotographic copy machine in which the belt moves past a number of processing stations, including a charging station and a developing station, for forming an image on the belt, and a transfer station for transferring the image to a receiving material. In electrophotographic copying machines of this type which have a photoconductive belt with a seam at the jointure of its two ends, it is well known that care must be taken to prevent image formation from taking place on the seam of the belt. See, e.g. IBM Technical Disclosure Bulletin, pp. 1526-27, Vol. 9, No. 11 (1967), Xerox Disclosure Journal, p. 505, Vol. 8, No. 6 (1983).
For example, in U.S. Pat. No. 3,976,375 the surface of the photoconductive belt is divided up into a number of equal large portions and into a second number of equal small portions. A predetermined section of a large imaging portion or a predetermined section of a small imaging portion is always used to form large or small images, respectively. The belt portions outside the large imaging sections are not used for forming large size images and the belt portions outside the small imaging sections are not used for forming small images.
The imaging sections are distributed over the belt surface so that the seam is situated outside both the large imaging sections and the small imaging sections. With a distribution of this kind some portions of the belt belong to both a small and a large imaging section, while other portions belong only to a large or to a small or to no single imaging section. However, this arrangement has the disadvantage that within an imaging section, the photoconductive belt will have a nonuniform frequency of image forming use everywhere therein.
As is known from U.S. Pat. No. 4,375,330, the photosensitivity of photoconductors decreases as a result of image forming use. In systems such as that described above, the photosensitivity of the belt does not remain the same everywhere within an imaging section because of the nonuniform frequency of use. In the course of time, this results in an inconsistent quality in the copies made requiring that the photoconductive belt be replaced prematurely.
As is apparent from the foregoing, the nonuniformity is the result of using imaging sections of different sizes. It is known that this nonuniformity or inequality can be prevented by using large imaging sections for forming both large size copies and small size copies wherein the imaging section being used is charged, exposed and developed in its entirety, irrespective of the size of the required copy, but only a portion, depending on the copy size, being transferred to the receiving material. This method, however, has the disadvantage that that portion used for making a small size copy is the same size as that used for making large size copies. One of the results of this is that the copying speed (number of copies per unit of time) for all copy sizes is equal to the low speed customarily needed for large copy sizes. This is a particular disadvantage in the case of copying machines used for copying working drawings in which the difference between the largest copy size (e.g. A0) and the smallest copy size (e.g. A4) is so great that the copying speed is unacceptably low for A4 copies.
Accordingly, it is an object of the present invention to provide a method without the above disadvantages, and to provide a device for enabling said method to be used therein.